1. Field of the Invention
In general, the present invention relates to a display apparatus, a driving method provided for the display apparatus and an electronic instrument employing the display apparatus. In particular, the present invention relates to a display apparatus having the type of a flat panel employing pixel circuits laid out 2-dimensionally to form a matrix as pixels each including an electro optical device and relates to a method provided for driving the display apparatus as well as an electronic instrument employing the display apparatus.
2. Description of the Related Art
In recent years, in the field of display apparatus for displaying images, a display apparatus having the type of a flat panel employing pixel circuits laid out 2-dimensionally to form a matrix as pixel circuits each including an electro optical device serving as a light emitting device has been becoming popular at a high pace. The electro optical device employed in each pixel circuit of a flat-panel display apparatus is a light emitting device of the so-called current-driven type in which the luminance of light emitted by the light emitting device varies in accordance with the magnitude of a driving current flowing through the device. An example of a flat-panel display apparatus employing pixel circuits each including a light emitting device of the so-called current-driven type is an organic EL (Electro Luminescence) display apparatus employing pixel circuits each including an organic EL device serving as a light emitting device. An organic EL display apparatus employs pixel circuits each including an organic EL device each making use of a phenomenon in which light is generated when an electric field is applied to an organic thin film of the organic EL device.
An organic EL display apparatus employing pixel circuits each including an organic EL device serving as an electro optical device has the following characteristics. An organic EL device has a low power consumption since the device is capable of operating even if the device is driven by an applied voltage set at a low level not exceeding 10 V. In addition, since an organic EL device is a device generating light by itself, an image generated by the light exhibits a high degree of recognizability in comparison with a liquid-crystal display apparatus displaying an image in accordance with an operation to control the luminance of light generated by a light source known as a backlight for a liquid crystal employed in every pixel circuit. On top of that, since an organic EL display apparatus does not desire an illumination member such as a backlight, the apparatus can be made light and thin with ease. Moreover, since an organic EL device has a very short response time of about few microseconds, no residual image is generated at a display time.
Much like a liquid-crystal display apparatus, the organic EL display apparatus can adopt either a simple (passive) or active matrix method as its driving method. However, even though a display apparatus adopting the passive matrix method has a simple structure, the light emission period of the electro optical device decreases as the number of scan lines (that is, the number of pixel circuits) increases. Thus, the organic EL display apparatus raises a problem of difficulties in implementing a large-size and high-definition model.
For the reason described above, display apparatus adopting the active matrix method are developed extensively in recent years. In accordance with the active matrix method, an active device for controlling a driving current flowing through an electro optical device is provided in the same pixel circuit as the electro optical device. An example of the active device is a field effect transistor of the insulated-gate type. The field effect transistor of the insulated-gate type is generally a TFT (Thin Film Transistor). In a display apparatus adopting the active matrix method, each electro optical device is capable of sustaining the state of emitting light throughout the period of one frame. It is thus easy to implement a large-size and high-definition display apparatus adopting the active matrix method.
By the way, an I-V characteristic exhibited by the organic EL device as a characteristic representing a relation between a voltage applied to the device and a driving current flowing to the device as a result of applying the voltage thereto generally deteriorates with the lapse of time as is commonly known. The deterioration with the lapse of time is also referred to as time degradation. In a pixel circuit employing a TFT of the N-channel type as a device driving transistor for generating a driving current flowing to the organic EL device included in the pixel circuit, the source electrode of the TFT is connected to the organic EL device. Thus, due to the time degradation of the I-V characteristic exhibited by the organic EL device, a voltage Vgs applied between the gate and source electrodes of the device driving transistor changes and, as a result, the luminance of light emitted by the organic EL device also changes as well. In the following description, the technical term ‘device driving transistor’ is used to imply a TFT for generating a driving current flowing to the organic EL device.
What has been described above is explained more concretely as follows. An electric potential appearing on the source gate of a device driving transistor is determined by the operating point of the device driving transistor and the organic EL device. Due to the time degradation of the I-V characteristic of the organic EL device, the operating point of the device driving transistor and the organic EL device changes undesirably. Thus, even if the voltage applied to the gate electrode of the device driving transistor remains unchanged, the electric potential appearing on the source gate of a device driving transistor changes. That is, the voltage Vgs applied between the gate and source electrodes of the device driving transistor changes. Thus, a driving current flowing through the device driving transistor also changes as well. As a result, a driving current flowing through the organic EL device also changes so that the luminance of light emitted by the organic EL device varies even if the voltage applied to the gate electrode of the device driving transistor remains unchanged.
In addition, in a pixel circuit employing a poly-silicon TFT as the device driving transistor, besides the time degradation of the I-V characteristic of the organic EL device, the threshold voltage Vth of the device driving transistor and the mobility μ of a semiconductor thin film composing a channel in the device driving transistor also change due to the time degradation. In the following description, the mobility μ of a semiconductor thin film composing a channel in the device driving transistor is referred to simply as the mobility μ of the device driving transistor. In addition, the threshold voltage Vth and the mobility μ which represent the characteristics of the device driving transistor also change from pixel to pixel due to variations in manufacturing process. That is, the characteristics of the device driving transistor vary from pixel to pixel.
If the threshold voltage Vth and mobility μ of the device driving transistor change from pixel to pixel due to variations in manufacturing process and/or due to the time degradation, the driving current flowing through the device driving transistor also changes from pixel to pixel as well even if the voltage applied between the gate and source electrodes of the device driving transistor remains unchanged. Thus, even if the voltage applied between the gate and source electrodes of the device driving transistor remains unchanged, the luminance of light emitted by the organic EL device also varies from pixel to pixel as well. As a result, screen uniformity is lost.
In order to sustain the luminance of light emitted by the organic EL device at a constant value not affected by variations of the I-V characteristic of the organic EL device, variations of the threshold voltage Vth of the device driving transistor and variations of the mobility μ of the device driving transistor for a constant voltage applied between the gate and source electrodes of the device driving transistor even if the I-V characteristic of the organic EL device, the threshold voltage Vth and the mobility μ change due to the time degradation, as disclosed in Japanese Patent Laid-open No. 2006-133542, it is thus necessary to provide a configuration including a variety of compensation functions.
The compensation functions of each pixel circuit include a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the I-V characteristic of the organic EL device, a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the threshold voltage Vth of the device driving transistor and a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the mobility μ of the device driving transistor. In the following description, the process of compensating the luminance of light emitted by the organic EL device for variations of the threshold voltage Vth of the device driving transistor is referred to as a threshold-voltage compensation process whereas the process of compensating the luminance of light emitted by the organic EL device for variations of the mobility μ of the device driving transistor is referred to as a mobility compensation process.
By providing each pixel circuit with a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the I-V characteristic of the organic EL device, a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the threshold voltage Vth of the device driving transistor and a compensation function for compensating the luminance of light emitted by the organic EL device for variations of the mobility μ of the device driving transistor as described above, it is possible to sustain the luminance of light emitted by the organic EL device at a constant value not affected by variations of the I-V characteristic of the organic EL device, variations of the threshold voltage Vth and variations of the mobility μ of the device driving transistor for a constant voltage applied between the gate and source electrodes of the device driving transistor even if the I-V characteristic of the organic EL device changes due to the time degradation whereas the threshold voltage Vth and the mobility μ change due to the time degradation and/or variations in manufacturing process. However, the number of components employed in every pixel circuit increases. Therefore, there are raised problems of difficulties to reduce the size of the pixel circuit due to the increased number of components employed in every pixel circuit and, thus, difficulties to implement a high-definition display apparatus.
In the mean time, as an example, there has also been proposed a pixel circuit capable of changing a power-supply electric potential appearing on a power-supply line for providing a driving current to the device driving transistor. Since the power-supply electric potential appearing on a power-supply line for providing a driving current to the device driving transistor can be changed, the pixel circuit does not desire a transistor for controlling transitions from a light emission period of the electro optical device to a no-light emission period of the electro optical device and vice versa. As a matter of fact, the pixel circuit also does not desire a transistor for initializing an electric potential appearing on the source electrode of the device driving transistor and a transistor for initializing an electric potential appearing on the gate electrode of the device driving transistor. For more information on the proposed pixel circuit, the reader is suggested to refer to documents such as Japanese Patent Laid-open No. 2007-310311. Since the transistor for controlling the transitions from a light emission period of the electro optical device to a no-light emission period of the electro optical device and vice versa and the transistors for initializing the electric potentials appearing on the source and gate electrodes of the device driving transistor can be omitted, the number of components employed in every pixel circuit and the number of wires connecting such components can be reduced.